Lithothamnion on Cliff Faces (Photo: Paul Tyler)
The walls inside this St Kildan cave look at first like they are covered in pink paint. In fact they are encrusted in a layer of Lithothamnion or 'Pink Chalky Weed'. Resistant to grazing, it survives in rockpools and is found on almost all rocky surfaces underwater. Although it doesn't normally live above the tide line it can survive in places where it is constantly kept wet - in rockpools and on the walls of sea caves, especially in exposed caves on the West coast and around St Kilda. During hot dry spells rockpools may dry out, whereupon the pink weed dies, turning from pink to white.
Lithothamnion below Kelp (Photo: Paul Tyler)
Grazed rock faces underwater are almost always covered in a thin layer of pink weed.
by Paul Tyler